docs: def insert(self, bs, pos=None):
          """Insert bs at bit position pos.
  
          bs -- The bitstring to insert.
          pos -- The bit position to insert at.
  
          Raises ValueError if pos < 0 or pos > self.len.
  
          """
          bs = Bits(bs)
          if not bs.len:
              return self
          if bs is self:
              bs = self.__copy__()
          if pos is None:
              try:
                  pos = self._pos
              except AttributeError:
                  raise TypeError("insert require a bit position for this type.")
          if pos < 0:
              pos += self.len
          if not 0 <= pos <= self.len:
              raise ValueError("Invalid insert position.")
          self._insert(bs, pos)